1. Technical Field
The present invention generally relates to electrodes for lithium batteries and lithium batteries.
2. Related Art
Lithium batteries using lithium or lithium containing material as negative electrodes are not only light in weight and large in capacity, but also capable of providing high voltages when combined with appropriate positive electrodes. For this reason, lithium batteries are widely used as batteries for portable electronic equipment, cameras, watches, electric tools, hybrid automobiles and the like. However, such lithium batteries use highly active lithium and organic electrolyte, which cause for concern about their dangers, such as, firing and explosion at the time of short-circuits. Therefore, it is an important issue in designing lithium batteries to secure their safety.
As a method for securing the safety, the use of non-aqueous electrolyte may be enumerated. As an attempt to implement the non-aqueous electrolyte, lithium polymer batteries that use gel polymer electrolyte have been developed. However, in this attempted method, organic electrolyte is impregnated in gel polymer, such that the problems concerning the limitation of battery cycle life and danger of explosion have not been solved.
Also, as another attempt to implement the non-aqueous electrolyte, lithium batteries that use ceramic electrolyte (inorganic solid electrolyte) have been developed (see, for example, Japanese Laid-open Patent Application 2006-277997 (Patent Document 1)). By using the ceramic electrolyte, only lithium ions are ions that move in the electrolyte by the battery reaction and therefore almost no side reaction occurs. Further, as the ceramic electrolyte does not include flammable organic solvent, and thus does not need a sealing member or a liquid sealing structure, whereby reduction in size and thickness of the battery can be achieved.
However, according to the method for powder molding ceramic electrolyte powder with electrode active material powder, as described in Patent Document 1 as one of the embodiments, insufficient contacts occur between the ceramic electrolyte powder and the electrode active material particles, and between the ceramic electrolyte powder and the ceramic electrolyte powder, such that battery power output high enough for practical use cannot be obtained. Furthermore, the interfacial contacts become unstable because of volume changes that take place with charge-discharge cycles, whereby the battery cycle life is deteriorated.
On the other hand, another type of lithium batteries has been reported (see, for example, Japanese Laid-open Patent Application 2004-179158 (Patent Document 2)) in which a vapor phase thin film deposition method such as sputtering is used to form a laminate of layers, such as, positive electrode thin film/ceramic electrolyte thin film/negative electrode thin film. According to such a method of laminating thin films, good contact can be achieved at interfaces between the electrode (the positive electrode thin film or the negative electrode thin film) and the ceramic electrolyte, and the thickness of the active material layer and the electrolyte layer can be reduced, such that high power output and excellent battery cycle life characteristics are expected to be obtained.
However, according to the method recited in Patent Document 2, the total thickness of the active material per unit area is only about 1 μm to several μm, and therefore it is difficult to manufacture batteries with sufficient charge capacity. In other words, in order to obtain a sufficient charge capacity as batteries, it is necessary to manufacture batteries in which the total thickness of active material exceeds 100 μm. However, by the method described in Patent Document 2, it is difficult to obtain the total thickness exceeding 100 μm, such that batteries with sufficient charge capacity have not yet been manufactured.